Protective helmets having hard outer shells for use in various military, industrial or other applications are well known in the art. In such helmets, it is generally desirable to provide a resilient liner assembly between the outer shell and the wearer's head to help absorb shock. While straps or similar elements have customarily been used in the past for this purpose, they must be adjustable to accomodate various head sizes, resulting in some wobbling from front to back or from side to side.
Various proposals for custom-fitted liner assemblies have been suggested in an attempt to overcome this defect. According to one known method of making a custom-fitted helmet, disclosed in Morton U.S. Pat. No. 3,882,546, the outer helmet shell is spaced a suitable distance from the wearer's head and foam is injected into the region between the outer shell and an elastic layer closely overlying the wearer's head. The necessity of directly handling the foaming agent limits the utility of this method in the field.
According to another method of making a custom-fitted helmet, disclosed in Chisum U.S. Pat. No. 4,100,320, the helmet liner is preformed with a plurality of adjacent pairs of cells respectively containing the first and second components of a foamable mixture. After the liner is placed between the helmet shell and the wearer's head, the cell partitions separating the first and second components are removed to initiate the foaming process. While this method avoids direct exposure to the liner foam, the complexity and hence expense of the preformed liner limit its practical application. Both of those methods, moreover, are one-shot procedures in that they do not permit subsequent adjustment of the liner to accommodate a different wearer or a changed head size.
Yet another method is disclosed in the commonly assigned application of Michael R. Lavender, Ser. No. 132,817, filed Mar. 24, 1980, now abandoned in favor of continuation application Ser. No. 382,420, filed May 27, 1982. That application discloses an individually fitted helmet liner having a plurality of layers, each of which consists of a thermoplastic sheet formed with an array of pockets which individually receive hollow epoxy balloon spacer elements. Adjacent layers are arranged with the spacer elements of one layer in register with the spaces between the elements of an adjacent layer, so that the layers nestle together to an extent determined by the degree to which the sheets are permanently deformed in the regions of the spheres of adjacent layers. The sheets making up the liner are elastic at normal temperatures but are plastically deformable at elevated temperatures to permit custom fitting to a changed head size simply by fitting the helmet after heating the layers to a suitable softening temperature.
While the helmet liner described above fulfills the objects of its inventor, there remain certain areas for improvement. First, the necessity of arranging the adjacent layers with the spheres of one layer in register with the spaces between the spheres of an adjacent layer entails a relatively expensive and time-consuming manufacturing step of maintaining the various layers in proper register. Second, the relative incompressibility of the hollow epoxy spheres results in a tendency of the completed helmet to shift its position relative to the wearer's head, owing to an inability of the liner to conform fully to the contours of the wearer's head. Finally, drawstrings or the like are required to maintain the sheets in tension during size adjustment.